Conscripted at the start of the Second World War, Sartre was taken prisoner by the German advance of 1940. He may have been released on medical grounds, he may have escaped, but by spring 1941 he was back in Paris where he founded the resistance movement Socialism and Freedom. All this time, invigorated by the war, he had been writing his major work, Being and Nothingness: An essay on phenomenological ontology, published in 1943.

Often called "the bible of existentialism", this dense 650-page book was the extraordinary distillation of everything his monumental intellect had read, written, considered, experienced and discussed for more than twenty years. Today it is part of the canon of Western philosophy.

"Sartre's question in Being and Nothingness", Cox continues, "is the same as that of his major influences, Hegel, Husserl and Heidegger: what is consciousness?"

What is the nature of a being that has and is a relationship to the world, that is an awareness or consciousness of the world and which acts upon the world? Sartre's answer is that the only kind of being that can exist in this way is one that is, in itself, nothing; a being that is a negation, non-being or nothingness.

Following Husserl, Sartre argues that consciousness is always consciousness of something. Consciousness is not a thing in its own right but entirely a relationship to the world it is conscious of. This is the theory of intentionality. Consciousness always intends its object and is never merely a set of brain states.

"Existentialism," he writes, "is best known as a philosophy of freedom:"

Sartre argues that freedom is limitless. This is often misunderstood. He does not mean we are free to jump to the moon, or that we can radically re-invent ourselves from scratch at any moment - but rather that there is no limit to our obligation to choose who we are through what we do or not do. This is what he means when he says we are "Condemned to be free". [...]

Post-war, Sartre developed his existentialism in an increasingly political direction. He placed his existentialist theory of the individual at the heart of the Marxist theory of the historically defined collective.